Barometric pressure varies with weather conditions and altitude. IN a motor vehicle, an accurate determination of barometric pressure is essential for various engine control functions. For instance, precise metering of the amount of air and fuel delivered to the engine is necessary to achieve the desired combustion as well as acceptable vehicle emissions. When the barometric pressure drops, typically the timing needs to be retarded and the fuel mixture richened. In addition, the barometric pressure may also be used to control idle bypass airflow, check for limp-in conditions and other diagnostic functions.
Barometric pressure can be measured in a variety of ways. Currently, in automotive applications, the barometric pressure can be measured using a barometric pressure sensor mountable on any suitable place on the vehicle where it sees true atmospheric pressure. Such a sensor generates an output signal indicative of the atmospheric pressure. The barometric pressure reading is then used for the various engine control functions. However, barometric pressure sensors can be costly and it is always desirable, particularly in automotive applications, to minimize costs.
Consequently, methods have been developed for estimating barometric pressure without the use of a separate or dedicated barometric pressure sensor. It is know, for example, that barometric pressure can be estimated when the vehicle's throttle is wide open (i.e., WOT) and, in some cases, when the vehicle's throttle is at some part throttle positions using an existing manifold absolute pressure sensor. However, there is typically a lower throttle position threshold below which barometric pressure cannot be estimated reliably. IN a hybrid vehicle, the accelerator pedal is not typically coupled to the throttle. Since the operator doesn't directly control the throttle, operation at WOT may not occur when needed or prolonged operation at or below the lower throttle position threshold is quite possible, thereby resulting in unreliable barometric pressure estimates.
Thus, it is desirable to have a reliable method for determining barometric pressure in a hybrid vehicle without the need for a barometric pressure sensor.